The present invention relates to an AM stereo demodulator, and more particularly, to a system for demodulating a quadrature-modulated AM stereo signal.
A conventional system of the same general type to which the present invention pertains is shown in FIG. 1, wherein reference numeral 1 is a divider, 2 and 3 are multipliers that perform coherent detection and respectively provide demodulated left- and right-channel signals, 4 is a limiter that removes all amplitude variations from the received AM stereo signal, 5 is a cosine detector that extracts the cosine components from the output of the limiter, 6 and 7 are switches that perform selection of the proper reference signal for coherent detection as one of the multiplication inputs to the multipliers 2 and 3; 8 is a phase-locked loop (PLL) for obtaining a reference signal for coherent detection form the AM stereo signal; 9 and 10 are phase shifters that respectively change the phase of the output of PLL 8 by +.pi./4 and -.pi./4, 11 is a lock detector for the PLL 8, 12 is an IF level detector for use in the detection of the intensity of an electric field, and 13 is an OR gate that receives the output of the lock detector 11 and that of the level detector 12 as two inputs and which controls the operation of the switches 6 and 7 in accordance therewith.
If an AM stereo signal .upsilon.(t) is generated by the Motorola system, it is expressed by: ##EQU1## wherein A and k are constants, L and R are left- and right-channel signals, and .omega..sub.c is a carrier angular frequency. Rewriting Eq. (1): ##EQU2## The AM stereo signal .upsilon.(t) expressed by Eq. (2) is applied to the limiter 4 and PLL 8, which is response deliver outputs to the cos .phi. detector 5 for detection of the cos .phi. component in each output. Therefore, in the divider 1, the AM stereo signal .upsilon.(t) expressed by Eq. (2) is divided by cos .phi. to eliminate the cos .phi. component.
The reference signal obtained from the PLL circiut 8 is fed to the phase shifters 9 and 10, wherein the phase of the signal is changed by +.pi./4 and -.pi./4 to produce two reference signals, cos (.omega..sub.c t+.pi./4) and cos (.omega..sub.c t-.pi./4), which differ in phase by 90.degree.. These two reference signals are multiplied by the output of the divider 1 in the multipliers 2 and 3. The division output of the divider 1 may be expressed by: ##EQU3##
Multiplying Eq. (3) by each of the two reference signals, cos (.omega..sub.c t+.pi./4) and cos (.omega..sub.c t-.pi./4), coherent detection of the L and R components is realized.
It is necessary to maintain the S/N ratio at a high level even if the intensity of the received electric field is reduced or the PLL 8 is in the unlocked condition. In order to meet this requirement, the switches 6 and 7 are controlled by the outputs of the lock detectors 11 and 12 so that the output of the limiter 4, rather than the outputs of the phase shifters 9 and 10, is fed as a reference signal to the multiplication input of the multipliers 2 and 3. As a result, an envelope detection output is produced by each multiplier, and a change from a stereo to monaural mode is realized to provide an improved S/N ratio.
However, with the conventional system described above, the change from the stereo to the monaural mode is so abrupt that the listener will sense some awkwardness in sound reproduction.